The present invention relates to an engine control apparatus including an interpolation control means, and especially to an engine control apparatus including an interpolation control means such that, when the required operational state is switched between one state to another state, a desired value of a controlled variable can be smoothly changed from a desired value of the controlled variable in the one state to that in another state.
In engine control, it is necessary to switch an operational state of an engine in response to an operational requirement corresponding to a change in a running state of a vehicle with the engine. In the switching of an operational state, since a desired value of a controlled variable in one operational state differs from that in another operational state, the switching of an operational state of an engine has been generally performed by searching a prepared map and so on in which the relationship between a desired value of a controlled variable and each operational state of the engine is described.
The above engine control is explained below by taking up a fuel burning (referred to simply as burning) control in an engine as an example. In a direct injection engine, a lean-burn operation is performed. In the lean-burn operation, a stoichiometric mixture burning in the vicinity of the stoichiometric air to fuel ratio (homogeneous charge burning) or a lean air to fuel ratio mixture burning (stratified charge burning) is selected corresponding to a change of an operational state such as a change in a required load for an engine.
In the above lean-burn operation, since fuel is burn at a superlean air to fuel ratio (hereafter, referred to as A/F), the fuel consumption cost can be greatly reduced, and the gas exhaust performance can be improved. However, in order to burn fuel at a superlean A/F in the engine, it is necessary to inject highly-pressurized fuel into each cylinder of the engine. Although the fuel pressure to inject fuel into cylinders of an engine is often set as constant, it is also required to change the fuel pressure corresponding to a fuel burning state from the point of view of sufficiently bring out the burning performance of the engine. In a conventional burning control, to satisfy the above requirement, a map search method has been generally used. This method uses a prepared map in which the relationship between a desired value of the fuel pressure and each burning state of the engine is described, and if the switching of a burning state is required, a desired value of fuel pressure is selected by searching the map, and the burning state is stepwise switched by changing the desired value of fuel pressure.
If the fuel pressure to inject fuel into cylinders of an engine is changed, when the required burning state is switched, the desired value of the fuel pressure should be smoothly changed from a desired value in the present burning state to that in another burning state, these desired values being described in a map. Such a burning control is especially required from the standpoint of the burning stability. As mentioned above, if the desired value of fuel pressure is stepwise changed, the actual fuel pressure also rapidly changes, which degrades the burning controllability, and may cause stopping of fuel burning or engine shock.